1. Field of the Invention
This invention relates to aeroacoustic noise reduction. More specifically, the invention is a system for reducing aeroacoustic noise originating at opposing side edges of trailing-edge flap components of aircraft high-lift systems.
2. Description of the Related Art
Conventional transport aircraft wing design is driven mainly by cruise efficiency, i.e., adequate lift is generated at high speed for level flight with minimal drag. Conventional high-lift systems using leading-edge slats and trailing-edge flaps were designed to augment lift and improve stall characteristics at the low speeds required during landing. These multi-element airfoil systems increase the effective chord (i.e., stream-wise dimension) of the wing and thus its effective area. The major effect of the multi-element airfoil arrangement is to generate a much larger pressure difference (lift) between the upper (suction) and lower (pressure) surfaces than would be possible via a single airfoil element.
The multi-element airfoil forms a smooth single-element profile during the cruise phase of flight to reduce wing drag. That is, the multiple airfoil elements are nested together in the retracted position. However, when deployed, the multi-element implementation of the high-lift system presents many discontinuities and other unfavorable, geometric features responsible for producing flow unsteadiness, and thus noise. The principal geometric features for producing flow unsteadiness at an airfoil's trailing edge are the side edges of flaps.
Existence of a strong pressure, differential between the bottom and top surface of the flap results in the formation of a complex dual-vortex system. More specifically, near the flap leading edge, the boundary layer on the bottom surface separates at the sharp corner and roils up to form the stronger of the two vortices. Similarly, the thin boundary layer on the side edge separates at the sharp top corner and forms what is initially the weaker of the two vortices. Both vortices gain strength and size along the flap chord because of the constant ingestion of vorticity. Downstream of the flap mid-chord, the side vortex begins to interact and merge with the vortex on the top surface. Eventually, a single dominant stream-wise vortex is formed. Considerable flow unsteadiness (i.e., noise source) is produced during the shear layer roil up, vortex formation, and vortex merging process as well as by the interaction of the vortices with the sharp corners at the flap edge.